Cross-platform memory map

[hawkw]

The params module should have a cross-platform way of representing the system's memory map, for populating the allocator and so on.

Rather than using the multiboot 2 memory map directly (since eventually we want to support systems other than multiboot), we should translate the memory map from multiboot into our own internal representation and use that instead,

[hawkw]

I believe this was more or less finished by fdf37bbb55b9c171ab6d4b36b6708b1dbb9046f0